1. Field of the Invention
The present invention relates to an electric energy storage element and more particularly, to a secondary cell and a capacitor using a quinoxaline compound as an electrode active substance and a proton as a charge carrier.
2. Description of Related Art
An electroconductive polymer has electroconductivity and also exhibits a reversible redox reaction upon electrochemical doping, and therefore, applications thereof have been considered not only to an electroconductive material but also to an active substance for a secondary cell and a capacitor. Particularly, in the field of cells, a strong demand exists to have a high energy density and a lightweight owing to the progress of miniaturization and weight saving of communication equipments in the field of information communication. A demand of a cell increases which uses a lightweight electroconductive polymer as an active substance.
As an electroconductive polymer exhibiting a redox reaction and being used as an electrode active substance, for example, polyaniline, polythiophene, polyacetylene, polyphenylene, polypyrrole, polytriphenylamine, polypyridine, polyquinoxaline and derivatives thereof are conventionally exemplified. It has been reported by Eui-hwan song, et al. in J. Electrochem. Soc., vol. 1, pp. 1193-1196 (1998) that polyphenylquinoxaline, as one of polyquinoxaline derivatives, exhibits electroconductivity in a strong acid (such as 37% nitric acid, 70% nitric acid, 99% trifluorosulfuric acid and 96% sulfuric acid) and carries out electrical charging and discharging. The inventors have reported in JP-A-2000-260422 that in the case where a polyphenylquinoxaline represented by the following general formula (1) (wherein n generally represents an integer of from 25 to 500) is used as an anode active substance, a secondary cell having a high energy efficiency and a high weight energy efficiency and a capacitor having a high capacity can be obtained. 
However, the energy density and the cycle characteristics are not in sufficient levels even in the case using polyphenylquinoxaline.
It is an object of the invention to provide an electric energy storage element which has a larger energy density than the case using polyphenylquinoxaline and has excellent cycle characteristics.
In the present invention, an electric energy storage element is defined by an element which can storage electric energy, that is, the element which can discharge electric energy toward an outer system and charge electric energy from the outer system, such as a secondary battery and a capacitor. According to the present invention, there is provided an electric energy storage element which contains a quinoxaline trimer compound as an electrode active substance and uses a proton as a charge carrier for the trimer compound. The quinoxaline trimer compound the timer compound consists of quinoxaline or a quinoxaline derivative.
It is preferred in the invention that the quinoxaline trimer compound has a bond between 2-positions of quinoxaline or a quinoxaline derivative.
It is also preferred in the invention that only an adsorption theory of proton of the quinoxaline trimer compound participates in electron donation and acceptance associated with a redox reaction of the quinoxaline trimer compound.
It is also preferred in the invention that the trimer compound is a compound represented by the following general formula (2): 
wherein R1, R2, R3, R4 and R5 each independently represents a hydrogen atom or an arbitrary substituent group. In the compound represented by the general formula (2), it is preferred that R1 represents a phenyl group, and at least one of the groups represented by R2, R3, R4 and R5 represents an electron donative group.
It is also preferred in the invention that the capacitor element contains an electrode containing the trimer compound in an amount of from 30 to 95% by weight, and it is also preferred that the capacitor element contains, as an electrolytic solution, a solution containing a proton in an amount of from 10xe2x88x923 to 18 mol/L.